In networks, especially networks with high real-time requirements, for example, in a head station (a so-called head end), in which a digital datastream with video and audio data associated in each case with different programs is generated for transmission within a digital radio system, the clock sources used are synchronized with a reference-clock source. In this context, the Global-Position-System (GPS)-time, which is broadcast via GPS-satellites as a high-precision time signal in the form of one-second pulses (1 Pulse Per Second (PPS)) or in the form of a 10 MHz reference clock, or the Network-Time-Protocol (NTP), which transmits reference-time information of the Universal-Time-Coordinated (UTC)-time from an NTP-server via a data-packet orientated network, preferably via the Internet upon demand from a requesting client, are used as the reference-clock source.
As shown in FIG. 1, timestamp information based on the reference time is transmitted in data packets from primary time-servers 11, 12, 13 and 14, the so-called NTP-servers, which are coupled with a high-precision reference-time source 21, 22, 23 and 24, the UTC reference-time source, upon demand from a secondary server 31, 32, 33 and 34. On the basis of the transmission time from the requesting, secondary server to the NTP-server and from the NTP-server to the requesting secondary server, which depends upon the level of data traffic in the Internet, the requesting, secondary server receives the requested time with a statistical time delay. This statistical time delay, which leads to an inaccuracy of the time in the requesting, secondary server, must be minimized as far as possible via appropriate algorithms.
One possibility for minimizing the inaccuracy of the requested time is implemented according to FIG. 2 in the case of several NTP-servers upon demand from the requesting clock filters 41, 42 and 43 of the secondary server 3. The minimum time, which corresponds to the smallest transmission delay, is selected from all of the received times in a sub-computer 5 of the secondary server 3 by means of an NTP-algorithm. Finally, NTP-servers in which the transmission time from the time request to reception of the time information is subject to major statistical fluctuations and/or the time information is basically false are eliminated.
A further minimization of this statistical time delay is realized by a phase-locked loop 6, a so-called phase-locked loop (PLL), integrated in the secondary server 3, in which the received times represent the clock pulses of the associated reference-time source counted in the NTP-server from an initialization time until the respective transmission time and therefore represent phases, are compared and corrected with the counted clock pulses of the associated reference-time source at the reception time of the individual timestamp information in the secondary server 3, which also represent phases. In the phase-locked loop, a minimization of this phase jitter or phase noise is implemented by averaging in an averaging filter over several phase differences. The clock number of the associated reference-time source counted in the secondary server at the reception time of the individual timestamp information is obtained from the received clock number with the addition of the clock pulses of the associated reference-time source counted in the time interval between the transmission time in the primary server and the reception time in the secondary server. The transmission time in the primary server is selected approximately, as a mean value between the time of the time request by the secondary server and the time of reception of the associated timestamp information in the secondary server.
A clock source for a head end, of which the clock pulse is also synchronized with the long-term stable reference clock of an NTP-server is known from DE 10 2009 057 362 A1. For the simultaneous increase of the short-term stability of the clock pulse of the clock source, the long-term stable reference clock of the NTP-server is interpolated with the short-term stable reference clock of a further reference-clock source.
Both the implementation of the averaging and also the interpolation requires a certain processing time within which a drift of the clock pulse of the clock source in the secondary server occurs, which leads to an accumulated time offset of the clock source of typically a few tens of milliseconds within a delay time of one hour. In this context, a change in the ambient temperature leads to a relatively rapid frequency change of the clock source, which is recognized and corrected by the averaging only subject to a delay. In the case of real-time applications of a head end, this can lead to an overflow or a zero loading of the buffer, in which the individual data packets of the digital transport datastream to be transmitted are buffered, and accordingly to a loss of data packets to be transmitted or a gap in transmission, which, in each case, represent unacceptable operating conditions.
What is needed, therefore, is an approach for rapid-response optimization of the short-term stability of a clock pulse of a clock source, which is already synchronized with a long-term stable reference clock.